1. Field of the Invention
This invention pertains to an apparatus for extracting pore fluids from porous solids to determine the suitability for a potential repository to store high-level radioactive wastes. Pore fluids are necessary for chemical analysis to help characterize the local hydrologic system and to evaluate the potential interaction of pore gas and water with waste canisters.
2. Background of the Prior Art
The U.S. Geological Survey (USGS), which has been conducting hydrologic site investigations at Yucca Mountain, Nev., has selected mechanical compression as the method for extracting pore fluid from unsaturated rock. The radioactive wastes would be placed within the thick section of unsaturated volcanic tuff. The physics of fluid flow in thick, fractured-rock unsaturated zones is not well understood. Established techniques are lacking for testing and evaluating this hydrological system. The use of chemical analysis of pore gas and water should help to better understand this hydrologic system.
The USGS, in a report "Triaxial-Compression Extraction of Pore Water from Unsaturated Tuff, Yucca Mountain, Nev." by In C. Yang et al., Water Resources Investigation Report 88-4189, U.S. Geological Survey, Denver, Colo., 1988, shows that tests performed with prior art equipment on similar samples produced less success than the instant invention. For example, testing failed to yield water from samples having water contents below 13 percent. The equipment was a biaxial stress chamber in which differential axial and lateral pressures were applied. The resulting stress state is less favorable to that imposed in a one-dimensional compression device since it induces high shearing stresses, which cause stiff particulate solids to dilate and/or rupture. Furthermore, gas injection was not used to recover residual water when the limits of mechanical compression were reached.
To extract any pore water, the stress levels must exceed the forces holding water within the pores. Therefore, only a certain range of compressive stress will yield a pore water that has suitable composition for chemical analyses. For example, one prior art study concluded that of the two adsorbed molecular water layers on a vermiculite clay, the water layer farthest away from the clay particle required 120 MPa hydrostatic stress for removal, whereas the closer water layer required 520 MPa hydrostatic stress for removal. These experimental extraction stresses matched predictions determined theoretically from water-adsorption curves of vermiculite. Another prior art test compressed sodium-bentonite clay and determined that there was an abrupt increase in the extraction of electrolyte-deficient adsorbed water at stresses greater than 59 MPa. They also concluded that the threshold for removing adsorbed water from sodium bentonite was a function of the dissolved-solids concentration of the pore water. When less mineralized or interstitial water with a minimal dissolved-solids concentration was used, smaller stresses affected the composition of the extracted water.
One-dimensional compression is not an uncommon tool for extracting pore fluids from porous solids, however, none of the prior art devices use bidirectional drainage or gas injection.